FIG. 1 of the accompanying drawings illustrates the stack structure of a typical liquid crystal display (LCD) module of small size, for example for a mobile phone or PDA device. The display includes a flat transmissive spatial light modulator (SLM) in the form of an LCD panel having input and output polarizers on its bottom and top sides. The rest of the structure is generally regarded as the backlight system, as follows. A light source (for example an LED or Laser) emits light, which is coupled into a lightguide and distributed across the back of the display by way of total internal reflection (TIR) in such a way that if no scattering structures were present the light would travel until it reached the end of the lightguide. Within the lightguide there are multiple scattering structures that extract the light from the lightguide to illuminate the LCD panel by disrupting the TIR conditions at the surface of the lightguide on which they are located, hence allowing the light to pass through the air-lightguide interface. These scattering features may be located on either the top or bottom major lightguide surfaces. The density of the light scattering features may increase with distance from the light source to maintain a uniform rate of extraction of the light along the length of the lightguide. As light is extracted both down and up from the lightguide, a reflecting film is placed beneath the light guide to improve the efficiency of the backlight. There are also some optical films between the lightguide and the LCD panel, placed to give better illumination uniformity over the display area and to enhance brightness within a given viewing angle range. These films typically consist of diffuser layers and prism films that enhance the central brightness of the backlight. The form of these structures is well known in the art and will not be discussed further here.
The form of the features that extract light from the lightguide is the main focus of the present invention. The typical form of extraction features involves “roughening” of the surface in some manner to disrupt total internal reflection (TIR) in the lightguide. The extraction in this case produces light that is emitted at a high angle to the lightguide normal and it preserves no coherence or polarization of the light.
There are many types of extraction features that can control the angle of extraction, for example U.S. Pat. No. 6,786,613 (Minebea) describes wedge shaped extraction features that extract light in a more vertical direction, but none of these types creates a polarized emission from an unpolarized source.
The amount of polarization of any light source is measured by the ratio of the electric field intensity in two orthogonal directions. These directions are known as TE (transverse electric) and TM (transverse magnetic). The ratio of the electric field intensities is known as the TE/TM ratio and is a measure of the level of polarization of a beam.
Conventional art related to polarized emission from a lightguide guiding unpolarized light is described below.
The advantage of a polarized backlight is that there is potentially no loss in the polarizers on the display, this significantly increasing the brightness of the LC display without increasing the backlight brightness. A backlight that produces a TE/TM ratio of substantially greater than 100 would be as good as the polarizers of the display, making them unnecessary. A backlight with a lower TE/TM ratio would still improve the loss from the polarizers.
Polarization sensitive interference films (“DBEF”) that reflect one polarization and transmit another are well known in the art. Commercially available versions typically produce a TE/TM ratio of approximately 3, limited by poor off-axis performance and absorption losses in the film.
US 2004/0246743 (Samsung Electronics Co.) describes a conventional rectangular grating printed on the bottom surface of a lightguide. The grating exhibits some polarization sensitivity by out-coupling more of the light belonging to one of the polarization states, with the polarization ratio of the transmitted field (TE/TM) depending on the amplitude (height) of the grating. However, polarization ratio TE/TM is not as high as “DBEF” films. Also, it works for limited angles of incidence and wavelengths.
U.S. Pat. No. 5,650,865 (Hughes Electronics) describes a holographic grating disposed on the top surface of a lightguide that transmits TE polarization and reflects TM. A phase retarding film deposited on the bottom lightguide surface gradually converts TM fields to TE, allowing for polarization recycling. Design is expensive and difficult to manufacture.
U.S. Pat. No. 6,688,751 (Slight Optoelectronics Co.) describes a backlight with a multilayer dielectric film deposited on its bottom surface. The dielectric film reflects light of one of the polarization states while it allows through the second state. Light reflected by the film is out-coupled from the lightguide while the polarization of the transmitted light is switched as it passes through a second film so that it can be reused. The design is expensive and very sensitive to the angle of incidence.
U.S. Pat. No. 5,258,871 (Eastman Kodak Company) describes a dual grating which allows for angular separation between TE and TM polarizations which are subsequently focused onto two different points. Design is aimed at projectors and not for panel LCDs.
M. Xu, H. P. Urbach and D. K. G. de Boer, “Simulations of birefringent gratings as polarizing color separator in backlight for flat-panel displays,” Opt. Express 15, 5789 (2007), describes a grating made of birefringent material deposited on a lightguide which transmits TE polarization and reflects TM. The design exhibits low TE/TM ratio.